In conventional spread spectrum communications systems, a difficulty exists in discriminating between the desired received communication signal and one or more interference signals which may also be received simultaneously therewith. Nulling techniques utilizing conventional adaptive nulling circuitry have been employed for minimizing the interference effects. Such current techiques utilize a transmitted reference or data decision signal which accompanies the originally transmitted communication signal in order to identify the communication signal at the receiver end. Alternative some current systems utilize an a priori knowledge of other characteristics of the desired waveform, such as the frequency hopping pattern thereof or the direction of arrival of the desired communication signal. Neither of such current approaches is a practical one for retrofitting of already existing antenna/receiver systems in order to provide the desired nulling capability for use with spread spectrum communications systems because existing systems may not have a transmitted reference signal available and the receiver in general is usually not equipped with a decision-directed mode of operation.
One further suggestion which has been used, for example, in spread spectrum communations systems which may be subject to some jamming or interference signals is to utilize two fixed antenna pairs, one with nulls in the forward and backward directions and one with nulls in directions orthogonal thereto. Each of the antenna systems comprises a pair of properly phased quarter wavelength spaced stubs, one pair of antennas, for example, at one location and the other at a separate location. The outputs of each antenna system would be connected to separate receivers with the best input being selected using suitable diversity techniques. Such an approach, however, seems to have limited capability, particularly where most of the potential jamming or interference signal angles of arrival are not adequately protected.
It is desirable, therefore, to develop a technique for providing some form of adaptive suppression of interference signals without the need for utilizing a transmitted reference signal or the need for other interfaces which require receiver or modem terminal modifications.